This invention relates broadly to the loading of thrust bearings and more particularly to a marine drive lower unit with sequentially loaded multiple thrust bearings.
Marine drives normally include an outboard positioned drive unit which includes a gear case for mounting a vertical main drive shaft connected at its upper end to a source of power, such as a marine engine. The lower portion of the drive shaft passes through a lower gear case portion which, in turn, is constructed in a manner to provide a generally horizontal torpedo housing. The housing serves to mount one or more propeller shafts which are driven by the main drive shaft and which drivingly rotate one or more marine propellers which are disposed generally aft of the gear case.
Heretofore, and in the case of a single propeller shaft, a pinion gear disposed on the lower end of the main drive shaft and within the housing has drivingly meshed with a driving gear mounted to the propeller shaft. In addition to the utilization of one or more reverse thrust bearings, a forward thrust bearing has been disposed between the driving gear and the housing to receive and carry the entire foward thrust on the propeller shaft caused by rotation of the propeller in a forward driving direction. Due to the major forces involved during forward driving, this thrust bearing has necessarily been of relatively high capacity, and thus of large diameter.
It is desirable to construct the torpedo housing to be of minimal diameter to thereby reduce drag as well as manufacturing costs. The previously known relatively large foward thrust bearing did not help in this regard.
It is an object of the present invention to provide a forward thrust bearing arrangement for a drive unit having one or more propeller shafts which enables a reduction in the diameter of the torpedo housing. It is a further object of the invention to provide an arrangement wherein a forward thrust bearing of large size is not required.
In accordance with the broad aspects of the invention, the thrust load on a shaft to be carried by two bearings is divided, utilizing a pre-load on one of the bearings. More particularly, and in the embodiment illustrated herein, a marine drive unit includes a lower torpedo housing. At least one propeller shaft is mounted in the housing for rotation about a drive axis. The propeller shaft is driven by a driving gear suitably connected to a marine engine and mounted on the drive axis. A first forward thrust bearing is disposed between the driving gear and the housing. In addition, a second forward thrust bearing is disposed adjacent the forward end of the propeller shaft. A pre-loading device, in the present embodiment a washer-like Belleville spring of a desired capacity, is disposed to provide an adjustable rearwardly biasing force on the second thrust bearing.
The construction is such that, when the drive unit is at rest, the second thrust bearing is adjustably preloaded and the first thrust bearing is free of load. As the drive unit is activated to rotate the propeller in a forward driving direction, the preliminary forward thrust carried through the propeller shaft is carried substantially exclusively by the second thrust bearing, until, and as the thrust increases, the pre-loading device finally permits the first thrust bearing to take the balance of the load. The result, after the sequential thrust take-up action, is to ultimately split the load between the thrust bearings, which therefore may each be of lower capacity and diameter.